Eric

Just as a basic thought: It is not the goal of a test to be as severe as possible but to proof that a product/process meets the requirements. To do so, the tests performed have to be chosen carefully and designed to show any week points but not to wreck the specimen on any account.

Now, you talk about different philosophies and different goals:

- Parallel testing: 
The idea of sequential testing is to find the answer of a specimen on a particular stress. There is no sense in starting combined tests is one doesn't know how an item to be tested reacts if it is exposed to let's say high temperature.

- Sequential testing: 
As I get it, you want to evaluate the effects of combined environmental stress on a specimen by exposing the specimen to the stresses one after the other. See, as soon as you get into combined testing the whole thing gets quite complex. Your thought is right, if the stresses do occur one after the other. One example is the degradation of roads due to frost. Firstly water penetrates into small cavities of the surface. When freezing the water expands breaking pieces out of the road. However, if the combined stress occurs simultaneously the attempt to simulate their combined effect by sequential testing seems fruitless to me. Just as an example for this thought we can take diffusion. As you know, diffusion through a membrane is driven by the temperature and by the difference of the concentration of a certain substance on both sides of the membrane. High temperature accelerates diffusion and so does a large difference in concentration. According to your thought one could theoretically perform a test where a specimen is firstly exposed to an environment with high temperature and a low difference in concentration dC ( or even worse not caring for the dC) and then making a test with the same specimen where it is exposed to a high dC with low Temperature (or similarly to above not caring for the temperature). As you see clearly, this test is absolutely meaningless when it comes to  find the effect of the combination of temp. and dC. If you want to find that you have to run tests where the dC and the high temperature occur together.

I think I mentioned this earlier. Designing sensible tests is a difficult business and requires a lot of knowledge of physics of failure and incorporates some math. To learn about the mathematical part read Montgomerys "Design and Analysis of Experiments". For the physics of failure part I am afraid you have to go through some literature dealing with physics, failure analysis and material science.


Have a great day 

Guenter

.

EMPA
Swiss Federal Institute for Materials Testing and Research
Centre for Reliability
Dipl. Eng. Guenter Grossmann

8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :     xx41 1823 4054
mail:     [log in to unmask]

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